Is President Trump right in saying there is "no evidence" his campaign team worked with Russia? Here's everything you need to know:

What exactly is collusion?"Collusion" is not a legal term, and it isn't a federal crime (except in antitrust law). What Special Counsel Robert Mueller's team is investigating is whether there was a conspiracy — "secret cooperation" between the Trump campaign and Russia in violation of one or more federal laws, including a law prohibiting political campaigns from receiving something of "value" from foreign nationals. The underlying question investigators are trying to answer is whether there was a simple quid pro quo: Russia helping Trump win the election in exchange for his using the power of the presidency to drop economic sanctions and adopt friendly policies toward the Kremlin.

What justifies that suspicion? U.S. intelligence agencies have already established that Russia sought to intervene in the 2016 presidential race to hurt Democrat Hillary Clinton and help Trump. During the race and after the election, the Trump campaign had an unusual amount of contact with Russians: at least 72 emails, phone calls, and other interactions, including 19 face-to-face meetings. In his sweeping investigation, Mueller is looking at a number of potential crimes: whether Trump campaign officials discussed with Russia the release of hacked Democratic emails, provided guidance for Moscow's disinformation campaign on social media, or accepted laundered campaign money or other help. Trump's prior financial dealings with Russians are also being examined as a possible source of influence or blackmail. So far, three Trump campaign officials have pled guilty to crimes related to Russia, and several aides, including Trump's son Donald Jr., are known to have expressed interest in receiving Russia's help. The key question, says former federal prosecutor Peter Zeidenberg, is "whether this interest crossed over into intentional solicitation."

What evidence is known? In June 2016, British publicist Rob Goldstone — who had extensive business ties in Russia — told Donald Trump Jr. in an email that as part of Moscow's "support" for his father, a Russian contact was offering "documents and information that would incriminate" Hillary Clinton. Trump Jr. responded enthusiastically — "If it's what you say I love it" — and forwarded the message to then–campaign manager Paul Manafort and Jared Kushner, Trump's son-in-law. The three men subsequently met in Trump Tower with Goldstone and four other people with Russian ties. U.S. sanctions against Russia were discussed, but Don Jr. insists the meeting produced nothing of interest or any follow-up contact. But even before that meeting, it's likely Trump's team knew Russia had potentially damaging "dirt" on Clinton and the Democrats.

How so? Earlier in 2016, a U.K.-based professor who had been cultivated by the Kremlin told Trump foreign policy adviser George Papadopoulos that Russia had "thousands of emails" that would damage Clinton if released. The young aide triumphantly repeated that claim to an Australian diplomat — whose subsequent tip-off sparked the FBI's investigation into the Trump campaign. Another key figure in this part of the investigation is Roger Stone, one of Trump's longtime advisers. Stone told a colleague in August 2016 that he had just met with WikiLeaks founder Julian Assange — a claim he later denied — and three weeks later tweeted that Clinton campaign chairman John Podesta would soon have his "time in the barrel." Shortly afterward, WikiLeaks began publishing Podesta's hacked emails. In an interesting bit of timing, the emails were released less than an hour after Trump's embarrassing Access Hollywood tape became public.

What other contacts occurred? Several Trump officials tried to set up back channels to Moscow during the post-election transition. Mike Flynn, Trump's first national security adviser, resigned when it was revealed that he had lied about discussing sanctions with then–Russian Ambassador Sergey Kislyak several weeks before Trump took office. In another message monitored by U.S. intelligence, Kislyak told his superiors that Kushner had proposed setting up a secret communications channel to the Kremlin in a Russian diplomatic facility. Mueller is also reportedly investigating contacts between Manafort aide Rick Gates and a former Russian intelligence officer, and a January 2017 meeting in the Seychelles between Kremlin-linked Russian financier Kirill Dmitriev and Blackwater founder Erik Prince, a Trump ally. Adding to suspicions is the degree to which Trump's aides have lied about their many Russian contacts.

What have they said? Top Trump campaign aide Jeff Sessions insisted during his confirmation hearings for attorney general that he "did not have communications with the Russians" during the campaign, only to later admit he'd met with Kislyak at least twice. In a statement reportedly dictated by the president, Trump Jr. initially claimed the topic of the Trump Tower meeting was "Russian adoptions," failing to mention the offer of "dirt" on Clinton. During the campaign, the senior Trump openly asked Russia for help, urging Moscow in a July 2016 press conference to "find" 30,000 Clinton emails. Does any of this mean the president was involved in a crime? Until Mueller releases his findings, says Brookings Institution senior fellow Benjamin Wittes, anyone who claims to know "is talking out of an orifice other than their mouth."

Was there a meeting in Prague?A controversial intelligence dossier compiled by former British spy Christopher Steele claims that Michael Cohen, Trump's personal lawyer, traveled to Prague around August 2016 to meet with Russian officials to plan a cover-up of their cooperation over hacked Democratic emails. Cohen denies that allegation, insisting he has never been to Prague "in his life." Trump defenders have cited Cohen's denial as proof that the entire dossier has no credibility. But McClatchy newspapers recently reported that Mueller has evidence Cohen did, in fact, travel to the Czech Republic to meet with Russians, entering via neighboring EU country Germany so that his passport bore no record of the visit. If the McClatchy report is true, it would mean Trump's longtime consigliere knowingly lied about a secret meeting with Russians — a major blow to the claim there was "no collusion." Federal agents recently raided Cohen's offices and home, carting away computers, tape recordings, and boxes of records.